In a process of a bonded SOI wafer for advanced radio frequency devices, a silicon substrate having a polycrystalline silicon layer formed on a surface layer thereof is used as a base wafer in some situations. A bond wafer is additionally prepared in addition to this base wafer, both the wafers are bonded, and then the bond wafer is thinned to fabricate the bonded SOI wafer.
Each of Patent Literature 1 and Patent Literature 2 describes a method for manufacturing a bonded SOI wafer for radio frequency devices which uses a polycrystalline silicon layer as a carrier trap layer (which is also referred to as a trap-rich layer). FIG. 6 shows a process flow of the method for manufacturing a bonded SOI wafer described in each of Patent Literature 1 and Patent Literature 2. They have a description that, as shown in FIG. 6, in the method for manufacturing a bonded SOI wafer, a polycrystalline silicon layer is deposited on a base wafer (S23), and then a surface of the polycrystalline silicon layer is polished (S24), and the base wafer is bonded to the bond wafer (S31). Further, FIG. 7 shows an example of a cross section of the bonded SOI wafer manufactured in this manner. In the bonded SOI wafer 1 shown in FIG. 7, a polycrystalline silicon layer 12, a buried oxide film layer (a BOX layer) 16, and an SOI layer 15 are formed on the base wafer 11 in the mentioned order.